1. Field of the Invention
The present invention relates to a system used in directional drilling and a related method of use and, more particularly, to such a system and method that uses a programmable digital computer to assist in controlling the drill azimuthal orientation of a string to control the drill bit trajectory.
2. Description of the Prior Art
The trajectory of a rotating drill bit through the earth's subterranean formations is partially determined by a plurality of drilling associated parameters, such as the mineralogy and geological configuration of the subterranean formations, the physical characteristics of the drilling equipment utilized, the type of drilling fluid utilized and the like. Further, the trajectory of the rotating drill bit is partially determined by the azimuthal orientation of the drillstring.
To provide the drilling operator with an understanding of the direction in which the wellbore is progressing, downhole measurement-while-drilling (MWD) tools, which are now commercially available, are utilized to send to the earth's surface for display to the drilling operator the operating parameters of the drill bit, such as RPM, weight-on-bit (WOB) and bit torque, and the wellbore's inclination and direction, usually measured as degrees from True North, and the azimuthal orientation of the drillstring.
That is, it is well known to those skilled in the art that when drilling with a downhole motor that forms a portion of the bottomhole assembly (BHA) which includes one or more axial bends in said assembly wherein the drillbit rotation is provided primarily by the downhole motor such that the drillstring extending from above the motor back to the surface is not rotated except for periodic adjustments to the azimuthal orientation of the bend(s) in the BHA that by applying a rotational force, either in a clockwise or counter clockwise direction, to an upper end of the drillstring it will cause the rotating drill bit connected at a lower end of the drillstring to move in the direction to which the rotation has been applied. This is generally called "steering" the bit.
The rotational force to the drillstring is applied by rotation of the drill rig's rotary table or by a hydraulic or electric power swivel, called a top drive. Initially, the azimuthal orientation of the drillstring is fixed at the surface to start the drill bit in the direction that is desired. Since the drillstring is an elastic system that is constrained at the surface by the drill rig's rotary table or top drive, the drill bit direction changes as the drill bit torque changes. These torque changes occur frequently because of the changes in the applied weight-on-bit (WOB), and changes in the subterranean formation's geological configuration. In order to keep the drill bit moving in the desired azimuthal direction, numerous rotational adjustments need to be applied on an almost constant basis.
Drillstring orientation is accomplished by rotating the rotary table or top drive in a stepwise manner, called "bumping," and/or adjusting the weight-on-bit (WOB). This stepwise rotation of the rotary table is time consuming and produces a gross adjustment. Any adjustments to weight-on-bit (WOB) often results in the drill bit operating at less than optimum weight-on-bit (WOB), which has been found to greatly reduce the drill bit's penetration rate. Additionally, the above described step wise adjustments produce a wellbore path that includes a series of crooked steps. Thus, wellbore completions, cementing, logging tool runs and the like can be more difficult or impossible.
There is a need for a method of directional drilling which can be used to substantially reduce the time involved in having to effectuate the changes in drillstring orientation, which will be done in a smooth manner so that a smoother wellbore path will occur all while maintaining a proper weight-on-bit (WOB).